pg_createsubscriber: Use new routine to retrieve data of PG_VERSION

pg_createsubscriber: Use new routine to retrieve data of PG_VERSION

pg_createsubscriber is documented as requiring the same major version as
the target clusters.  Attempting to use this tool on a cluster where the
control file version read does not match with the version compiled with
would lead to the following error message:
pg_createsubscriber: error: control file appears to be corrupt

This is confusing as the control file is correct: only the version
expected does not match.  This commit integrates pg_createsubscriber
with the facility added by cd0be131ba6f, where the contents of
PG_VERSION are read and compared with the value of PG_MAJORVERSION_NUM
expected by the tool.  This puts pg_createsubscriber in line with the
documentation, with a better error message when the version does not
match.

Author: Michael Paquier <[email protected]>
Reviewed-by: Masahiko Sawada <[email protected]>
Discussion: https://postgr.es/m/[email protected]